The trouble followed a parade of junior members of the Protestant Orange Order which passed along the lower end of the mainly nationalist Garvaghy Road on Saturday.

The RUC said groups of nationalist men and youths confronted officers on public order duty as they were withdrawing from the security operation mounted at the parade.

However, republicans criticised the police for firing plastic bullets and said the fighting was provoked by loyalists breaking parade restrictions and RUC heavy-handedness in trouble which followed.

Adam Ingram: "Riot was in no way a legitimate protest"

The rioting lasted two hours with up to 200 people involved.

At the height of the trouble, the police were attacked with petrol bombs, acid bombs, bricks and bottles by nationalists.

It is understood officers sustained a variety of injuries including concussion and broken bones. One has been detained in hospital.

Mr Ingram said those involved in the trouble were "not engaged in anything that could be remotely called legitimate protest".

'Escalating tensions'

Republicans said the annual march had a history of escalating tensions.

Dara O'Hagan: "Commission refused to listen"

They also said tensions were already high in the town following Celtic's victory in the Scottish Cup final on Saturday.

The march passed the bottom of the Garvaghy Road - as opposed to the top where Orangemen have been banned from marching to Drumcree Parish Church since 1998.

Garvaghy Road residents group spokesman Breandan MacCionnaith accused the police of being heavy handed and said several people were hit by plastic bullets.

Sinn Fein assembly member Dr Dara O'Hagan accused the Parades Commission of ignoring its appeals to re-route the Parkmore Junior Orange march further away from the Garvaghy Road.

"The Parades Commission refused to listen to this good advice and instead bowed to pressure from the Orange Order," she said.

Brid Rodgers has raised plastic bullet use with Irish Government

"The resulting use of 12 plastic bullets by the RUC once again demonstrates that the RUC are unable or unwilling to deal with situations where the Orange Order and the residents' group come into conflict fairly and evenly.

"We would urge not just the Parades Commission but also the Orange Order to think long and hard about how we move to resolve this issue."

SDLP assembly member Brid Rodgers said she was concerned that the police had reacted to the trouble by firing baton rounds.

She said: "I am very concerned at the rioting in Portadown last night and have raised the issue of the use of plastic bullets with the Irish Government."